Night Below: An Underdark Campaign is a boxed set by Carl Lynwood Sargent, for 2e in 1995. Noted in the Usenet group rec.games.frp.dnd for inducing severe depression on those playing through it on account everything is ugly, and damp, and wants your orifices for its pulsating turgid tentacles. It's pretty much Heart Of Darkness: The Game.
The set collects three books, each sixty four pages, amounting to an Adventure Path if they had been published separately.
The Evils of Haranshire
Haranshire is the generic countryside setting where eeevil lurks just beneath the surface, or in this case maybe a mile beneath the surface and HUNDREDS of miles southeast. The eeevil are kidnapping spellcasters, like one Jelleneth. Be prepared, as in Apocalypse Now, to not be reunited with this plot device for awhile.
Sargent must have realised that there wasn't much up here to do, so added a bunch of sidequests in the hope the party can get up to 5th level. Still isn't enough.
Perils of the Underdark
Here the party descends to the depths of the earth probably to get eaten by dozens of trolls a few steps in. Should the party manage past that, they'll face a Deepspawn. The book ends with the party playing the terrorist and conducting a genocide against a hapless kuo-toa city, the sad fishy fuckers. There is an end-goal here, so it's all more linear than Haranshire was; but a DM can flesh it out.
Here is Sargent's shadow-elves again, under another name and less dickish. There are also communities of quaggoth, svryfnenfb... fuggit, deep gnomes; and derro. Try not to kill everyone. Drow are periodically teased, but never met.
Sargent didn't bother advising his dungeon-masters when would be an appropriate level to face certain monsters, so they'll need to ask around for advice.
The Sunless Sea
Here in the fantasy genre's least-originally-titled setting the party goes against Great Shaboath, city of aquatic beasties, to destroy it and WHAT THE HELL WE JUST FOUGHT A CITY LIKE THIS. Well, this one is bigger. You can meet three sexy demonesses near the city, one of each demoness type available in the first two monster manuals.
The new monsters are: Savant aboleth, Rockseer Elf, ixzan ixitxachitl, giant moray eel, and Shaboath golem. Dragon magazine had already mooted superpowered aboleth, which Sargent didn't use. The skum race, available as of Forgotten Realms 2e, isn't here either. Guess you should set this in Greyhawk then.
Also here are sheets from the private journal of a previous band of suckers who wandered down here to get TPKed. They never met the nice elves.
And maps and illustrations and stuff like that.
Cliff Ramshaw squeed all over this, in the March 1996 Arcane, with a 9/10. Dragon Magazine #234 gave it a more sober 4/6.